Point-of-sale scanner windows, lenses, mirrors, viewing screens, transparent counter tops, and certain infrared devices, are some examples of the many applications in which a hard, transparent surface coating of good optical clarity is required as a protective layer for glass or other transparent material in order to protect them from mechanical damage due to wear, impact, scratching and the like. Such protective coatings must protect the underlying substrate material from damage, and be resistant to damage themselves. Damage may be defined as any scratch, mark or defect which causes any decrease in clarity of the coating through the scattering, deviation, or dispersion of light that would normally pass through undeflected. Transparent dielectric materials are those which are transparent in some portion of the electromagnetic spectrum, from 0.2 microns to 30 microns in wavelength.
Transparent dielectric coatings such as aluminum oxide, aluminum nitride, silicon nitride and others well known to workers in the field, are commonly used as mechanically durable transparent protective overcoats for softer materials such as glass and plastic, among others. In order for these and other hard protective coatings to function they must be sufficiently thick to resist cracking brought about by the elastic deflection of the softer underlying substrate material during a damage event such as a scratch or impact. While the coating thickness required to resist cracking due to substrate deflection i0 will depend upon the coating and substrate materials as well as the severity of the damage environment, it is generally held that such protective coatings must be greater than 0.5 micron in thickness, and may, in severe damage environments, be as thick as 100 microns.
The coatings required for transparent protective overcoats may be deposited by any of a number of deposition techniques, such as evaporation, sputtering, chemical vapor deposition, dip coating or others well known to workers in the field. Regardless of the method of growth or deposition of the coating it is found that the surface roughness of these coatings tends to increase with the thickness of the coating.
The surface roughness of these coatings is a consequence of the growth characteristics during deposition. In the initial stages of deposition, coatings tend to form as numerous separate islands on the substrate. As more coating material arrives at the substrate these islands grow laterally until they impinge upon one another. Once the entire surface is covered with an island structure the islands begin to grow in height, becoming columns. As the coating continues to grow in thickness all of the columns do not grow in height at exactly the same rate due to differences in column diameters and atomic arrangement within columns. These differences in column growth rates thus create a surface roughness which increases in magnitude with coating thickness.
There are other factors which contribute to surface roughness in coatings. So called growth nodules are formed when minute particles of dust or other particulate matter are incorporated into the coating during growth creating a surface bump or protrusion. Similarly, any roughness in the underlying substrate surface will tend to be replicated by the coating to produce a surface irregularity.
For very thin coatings, less than approximately 0.5 microns in thickness, growth nodules and substrate roughness are the main contributors to coating roughness. As coating thickness increases, the contribution to surface roughness caused by nodules and substrate roughness decreases while that due to columnar growth increases.